Take a piece of wood, and drill 27 5mm holes.Insert a LED into each hole.Take a Dymo labeler, print a few numbers and stick them to the board.Wire the LEDs, connect them to a PIC... That's all !

This example will show you how to connect up to 30 LEDs to a PIC, using only six I/O pins.

Circuit Schematic

I wanted to build a clock as simple as possible :

built around a little 18 pins PIC

no 7 segment display, only LEDs

no decoder, no buffer, no driver for the LED display

a cheap temperature sensor

The solution of direct LED driving comes from a Microchip Application Note AN234, and as I'm using 25 mA LEDs, I simply removed all current limiting resistors.A silicon diode is used as temperature sensor, and the rest of the circuit is very classic around a PIC16F819 :

Only the hours row is displayed : press ADVANCE button to change hour, and press VALID button when it is correct.The hours row is shut off, and the middle and lower rows of minutes are displayed : press ADVANCE to change minutes, and press VALID when it is correct.The clock starts with seconds set to zero.

To change display : press the MODE button to switch display in this order :